Submarine-cable signaling



June 1 1926.

- 1,586,895 J. J. GILBERT SUBMARINE CABLE SIGNALING Fil M y 1922 2Sheets-Sheet 1 //71//1/0n- John J. GI/b/f June 1 1926. v 1,586,895

J. J. GILBERT SUBMARINE'CABLE SIGNALING Filed May 15, 1922 2 SheetS-Sheet 2 W My tr l

Patented Tune l, was.

PATENT,

JOY-IN J'GILBERT, O1? PORT WASHINGTON, NEVT YORK, ASSIGNOR TO WESTERNELEC- TRIO COMPANY, INCORPORATED, 033 NEW YORK, N. Y., A GOBPORAIIGN OFNEW YORK.

SUBMARINE-CABLE srenanrne.

Application fiiana 15,

This invention relates to two-way operation oi high speed signalingconductors such as inductively loaded submarine cables.

An object of the invention is to provide a method and means for two-wayoperation over high speed signaling conductors, when it is diliicult tomaintain a balance between the conductor and an artificial linesutlicient to permit of duplex operation.

Another object is to provide a method and means for two-way operation oia signaling conductor by which the receivers are maintained out ofoperative relation with the conductor during transmitting intervals.

Ordinary submarine cables are not adapted to high speed signalingbecause of the excessive distortion produced by the large distributedcapacity. It has not been the practies to load long telegraph cableswith inductance toovercome the etlect of the capacity, although manysuggestions for such loading have been made. Until recently iron wasconsidered the only suitable material for inductive loading and itappears that it would be quite as effective to increase the Sif QG ol'the copper conductor as to load with iron. lt was doubtful, however, itthe improvemen effected by either expedient would justify the additionalexpense. There has recently been discovered, however, a nickeldronalloy, which has been appropriately termec permalloy, which has aremarkably high permeability for the low magnetizing forces utilized insignaling systems. This alloy, when properly prepared and made into aribbon of proper proportions and laid spirally upon a cable conductor inthe proper way, very greatly increases the permissible speed ofsignaling. For a more complete disclosure of this material and itsmethod of preparation, see U. S. application of G. W. lillmcn, S rialNo. 473,877, liled lv'lay 31, 1921. An obstacle to the use of suchloading has arisen, however, in the form of a failure of the artificialline, ordinarily employed in duplex operation, to maintain a sulhcientlyexact balance at the high available signaling speeds.

its is well ltnown the ordinary cable terminal circuit arrangementcomprises a socall duplex bridge, two of the ratio arms conting of equalimpedance such as con densers and the other two comprising the cable andan artificial line respectively. The

1922. Serial No. 561,138.

transmitter is ordinarily connected between the apex of this bridge andground and the receiver is connected in conjugate relation to thetransmitter. The artificial line is made up of sections of lumpedimpedance elements to simulate the impedance of the cable. The cableimpedance, however, varies from time to time due to temperature changes,earth currents, etc, so that an 0c casional readjustment of theartificial line is necessary. The higher. the signaling frequenciesemployed, the more diflicult it becomes to maintain an exact balance andat the signalingspeeds now available such a balance cannot be maintainedwith terminal apparatus ordinarily used.

This invention provides a method of duplex operation which does notdepend upon the conjugate relationship oi transmitter and receiver.Signals are transmitted over a submarine cable at relatively lowvelocity as compared with transmission over ordinary land lines so thatan appreciable period elapses between the time when the transmitting keyis closed and the arrival of the transmitted pulse at the receiver. Inits preferred form this invention consists. in si multaneouslytransmitting signals at both ends of the line during a period equal tothe time required for a signalto traverse the line and then connectingboth ends of the line to the receiving apparatus for an equal periodduring which the signals that were impressed upon the line during thetransmission period are received.

The invention will be understood from the following description and theaccompanying drawings, in which Fig. l is an arrival curve for a highspeed permalloy loaded cable; Fig. 2, a curve representing current lll'1ressed upon the cable at the transmitter; llig. 3, a signaling currentimpulse; Figs. 4 and 5, diagrammatic showings of two cmbodiments of theinvention; and Fig. 6 a lengthening network shown diagrannnatically at Lof Figs. 4 and 5.

Referring to Fig. l, a curve is plotted to show the relation betweenreceived current I and elapsed time T while an electromotive force isapplied to the cable at the transmitting station, as by the closure of akey. The received current is Zero during the definite interval 5:0 to tT and at the latter instant suddenly assumes the value i and increasesgradually to its final steady value. The quantities T and ,2 can bedetermined by the formulae:

T.=s. ot

in which S is the length of the cable, 6 is the base of the Naperiansystem of logarithms and R, LandG are the cable parameters, resistance,inductance, and capacitance per unit length of the cable. For thederivation of these formulae, see The theory of the submarine telegraphand telephone cable by H. V. Malcolm, 1917, page 51 The head of thearrival curve is not quite vertical owing to distortion produced byvariable resistance and dielectric losses.

The current I impressed upon the cable by a steady voltage appliedcontinuously is represented in Fig. 2. If this voltage s applied for ashort period of time only, as in the form of a dot signal the impressedcurrent takes the form shown in Fig.

It is seen that the current in the latter case consists of the signal a,'6 followed by a tail 5, c, which continues for a length of timedepending upon the constants of the cable and of the elements of thenetworks employed at the sending end for shaping the signal. It ispossible by proper design ot these networks to eliminate the tall or atleast reduce it so that, in an interval of time comparable with thelength of the signal a, 5, its value becomes small compared with themagnitude of a signal received from the distant end of the cable.

Referring to Fig. 4, which shows one embodiment of the invention, thecable C terminates at each end in two branch circuits containing thetransmitter and the receiver respectively. These branches arealternately associated with the cable by means of switches S, S, whichare maintained in synchronism by any well known means-for example, bymeans of the type commonly employed in printing telegraph systems. Theseswitches are shown as circular in form each comprising an insulatingsegment and a conducting segment, the brushes maintaining contact withthe periphery of these segments. The speed of rotation is such that thetransmitters are operatively connected with the cable for a certaindefinite period which is substantially the same as, or less than, thetime required for a transmitted signal pulse to arrive at the receiver.The transmitters are then disconnected from the cable by the switches S,S which then operatively associate the receivers with the cable for alike period. The transmitter employed is preferably automatic and may beof any well known form, such as a tape transmitter, and means areprovided for arresting the motion of the tape and associated apparatusduring the receiving periods. This mechanism may be associated with theshaft of the switch S by mechanical coupling G.

Reception may be by ear or by any suitable .1

mittal of a signal at the distant end of the cable at the time $20,there will be an interval T during which nothing will be received at thenear end of the cable. It is possible, therefore, without interferingwith the reception of the signals from the distant end of the cable, toemploy this interval for the transmission of signals at the near end ofthe cable, and the apparatus of Fig. 4 is designed to accomplish thispurpose. At the end of the period T,, however, the first of the signalswhich has been transmitted from the distant end will arrive, andtherefore at this time, or a little previous, it will be necessary todisconnect the transmitter, replacing it by apparatus for receiving thesignals which are about to arrive. Assuming that this procedure has beenadopted at both ends, it can be seen that the trains of received signalswill last for a period of time T immediately after which there will be agap of length T in the sending, due to the fact that the distantterminal has been changed over from transmitting to receiving. Theinterval of time required at each end of the cable between the lasttransmitted signal and the first received signal depends upon the timethat is taken by the tail of the former to fall to a value that will notinterfere with the received signals. The magnitude of the tail may bereduced, for example, by means of a transmitting condenser K of the kindordinarily used in sending end shaping, or the transmitting apparatusmay be inserted in a terminal bridge arrangement such as used inordinary duplex telegraphy, but with a degree of balance much inferiorto that required in the usual method of operation.

If a terminal bridge such as that mentioned in the last paragraph is tobe employed, the system may take the form shown in Fig. 5. The terminalbridge circuits are of the form ordinarily employed in duplex operation,but the artificial lines A L do not simulate the impedance of the cablewith sufficient accuracy to permit signals to be received at eitherterminal during transmis sion at that terminal. Signaling is accomplished by the method described above in connection with Fig. 4. Theswitches S,

.iitl

S alternately connect the transmitters and the receivers to the line fora period equal to or less than the propagation period T as in Fig. 4. Inplace-of the mechanical coupling G, G of Fig. i starting and stopping oftransmitting and recording mechanism may be controlled by auxiliaryelectric contacts as here shown. The artificial lines A L very closelysimulate the impedance ofthe cable for that portion of the transmittedcurrent whichtorms the tail of the signal, as shown in Fig. 3. This isthe portion of the transmitted signal current which is mosteasilybalanced and the eilect of which upon the local receiver may thereforebe eliminated.

It large irregularities of impedance occur along the cable, it may benecessary to balance the disturbances reflected from these This willentail corresponding changes in the switch arrangements at the cableterminals, which it might be inconvenient to malt It is thereforeproposed to place at each terminal in series with the cable alengthening network L, shown diagrammatically in Fig. 6, the purpose ofwhich is to increase the time required for propagaticn of a signalwithout unduly increasing the overall attenuation. This may comprise oneor more sections of network, each having an inductance Z and capacityl2. Additional need for the lengthening network may arise from the factthat the time T of the cable alone may not be suitable for the signalingspeed, for instance, T, might be equivalent to 2 ,4; cycles of thesignal speed in which case it would be desirable to increase T by anetwork until the total time of cable and network was 3 cycles.

Although this invention has been described with particular reference toa sub marine cable system it may obviously be used in other types ofsignaling systems.

lVhat is claimed is 1. The method of two-way signaling be tween twostations wiich comprises alternately simultaneously transmitting fromboth stations and receiving at both stations "for equal periods.

2. a method of two-way signaling over a submarine cable which comprisestransmitting simultaneously from both terminals for a period not greaterthan the time required for an initial pulse to reach'the receiver,ceasing to transmit at the end of said period at both terminals,receiving at both terminals for a like period and repeating in regularsuccession said steps of transmitting and receiving.

3. A method of two-way signaling over a submarine cable which comprisestransmitting simultaneously from both terminals for a periodsubstantially equal to the time required for an initial pulse to reachthe re ceiver, ceasing to transmit at the end of said period at bothterminals, receiving at both terminals for a like period and repeatingin regular succession said steps of transmitting and receiving.

4. The method of two-way signaling over a conductor which has anappreciable propagation period and is provided with means foroperatively associating it with both terminal transmitterssimultaneously and associating it from them and for operat-ivelyassociating said conductor with both terminal receivers during the timethat the transmitters are disassociated which method comprises operatingsaid means at a rate which will cause said associating anddisassociating to occur at intervals dependent upon said propagationperiod.

The combination with a submarine cable, of a transmitter at eachterminal thereof, means for causing said transmitters simultaneously toimpress current signal impulses upon said cable for a period not greaterthan the time required for a signal pulse to reach the receiver,automatic ID ans for rendering said transmitters inoperative to transmitsignal impulses ove' said cable during a period succeeding saidtransmitting period and of approximately the same length, and receivingmeans at each termi nal for receivingsaid signal impulses.

6. The combination with a submarine cable, of a transmitter at eachterminal thereof, a receiver at each terminal thereoi, and automaticmeans synchronously driven for alternately operatively associating saidtransmitters and receivers with said cable at a period not greater thanthe time required for a transmitted signal pulse to reach the receiver.

7. The combination with a submarine sicnaling cable, of a lengtheningnetworu therein, transmitting and receiving means at each terminal ofsaid cable, and automatic means synchronously driven tor alternatelyoperatively connecting said transmitting and receiving means with. saidcable at a period not greater than the time required for a transmittedsignal impulse to reach the receiver.

8, The combination with a submarine cable, or a transmitter at eachterminal thereof, means for causing said transn'iitters simultaneouslyto impress current signal impulses upon said cable for a periodsubstantially equal to the time required for a signal pulse to reach thereceiver. automatic means for rendering said transmitters inoperative totransmit signal impulses over said cable during a period succeeding saidtransmitting period and approximately the same length, and receivingmeans at each terminal for receiving said signal impulses.

9. The combination with a submarine cable, of a transmitter at eachterminal thereof, a receiver at each terminal thereof, and automaticmeans synchronously driven for operatively associating said transmittersand receivers with said cable at a period substantially equal. to thetime required for a transmitted signal pulse to reach the re ceiver.

10. The combination with a submarine signaling cable, of a lengtheningnetwork therein, transmitting and receiving means at each terminal ofsaid cable, and automatic means synchronously driven for alternatelyoperatively connecting said transmitting and receiving means with saidcable at a period substantially equal to the time required for atransmitted signal impulse to reach the receiver.

11,. The combination with a submarine signaling cable, of a lengtheningnetwork therein comprising series inductance and shunt capacity,transn'iitting and receiving means at automatic means for alternatelyoperatively connecting said transn'litting and receiving means with saidcable at a period not gremer than the time required for a transn'iittedsignal. impulse to reach the receiver.

12. The combination wit-l1 a submarine signaling cable, of alengtheningnetwork comprising a plurality of sections each of which contains seriesinductance and shunt capacity, transmitting and receiving means at eachterminal of said cable, and automatic means for alternately operativelyconnecting said transmitting and receiving means with said cable at aperiod not greater than the time required for a trans-.

mitted signal impulse to reach the receiver.

13. The combination with a. long signaling conductor having a signalpropagation period of appreciable length, of a duplex bridge arrangementat each terminal thereof comprising a receiver and a transmitter inconjugate arrangement with each other, an artifical line at eachterminal roughly simulating the impedance of said cable and automaticmeans for alternately operatively associating said transmitterssimultaneously with said conductor and dissociating'them therefrom andfor dissociating said receivers from said conductors during transmissionperiods, the periods of transmission being not greater than the signalpropagation period of said conductor.

14. The combination with along signaling conductor having a signalpropagation period of appreciable length, of a duplex bridge arrangementat each terminal thereeach terminal of said cable, and

of comprising a receiver and a transmitter in conjugate arrangement Witheach other, an artificial line at each terminal roughly simulating theimpedance of said cable but not producing a suiiiciently accuratebalance to provide for simultaneous transmission and reception at thesame terminal, and automatic means for alternately operativelyassociating said transmitters simultaneously with said conductor anddissociating them therefrom and for dissociating said receivers fromsaid conductors during transmission periods, the periods of transmissionbeing not greater than the signal propagation period of said conductor.

15. The combination with a-signaling conductor having a signalpropagation period of appreciable length, of transmitting and receivingmeans at each terminal thereof, auton'iatic means for operativelyassociating said transmitting means simultaneously at each terminal withsaid conductor and dissociating them from said conductor at a period notgreater than the signal propagation period of the conductor, and meansfor reducing the magnitude of the tails of the transi'nitted signals.

16. In a submarine cable system, means for rendering the transmitter ata terminal station ineii'ective and for sin'iultaneously andautomatically rendering the receiver at that station operative toreceive signals, and means eii'ective only for frequencies not greaterthan the signaling frequency for preventing the discharge of the cableafter cessation of transmission from interfering materially with thereception of signals.

17. In a submarine cable system, switching means for rendering thetransmitter at a terminal station ineffective to transmit signals, meansat that station for receiving 'incou'nng signals, and a balancingnetwork which roughly simulates the impedance of said cable forfrequencies not substantially greater than the signaling frequency forreducing the interference with received signals of the discharge of thecable after cessation of transmission.

18. In a. telegraph system, a cable and a pair of terminal stationsconnected to the ends thereof, each of said terminal stations comprisinga transmitting mechanism, a receiving mechanism and switching means atone terminal station for connecting the receiving mechanism to the cablesimul taneously with the receipt of a current impulse transmitted fromthe other terminal station.

In witness whereof, I hereunto subscribe my name this 12th day of May A.D. 1922.

JOHN J. GILBERT.

